EVP Symmetric Encryption and Decryption

The libcrypto library within OpenSSL provides functions for performing symmetric encryption and decryption operations across a wide range of algorithms and modes. This page walks you through the basics of performing a simple encryption and corresponding decryption operation.

In order to perform encryption/decryption you need to know:

Your algorithm

Your mode

Your key

Your Initialisation Vector (IV)

This page assumes that you know what all of these things mean. If you don't then please refer to Basics of Encryption.

Contents

The code below sets up the program. In this example we are going to take a simple message ("The quick brown fox jumps over the lazy dog"), and then encrypt it using a predefined key and IV. In this example the key and IV have been hard coded in - in a real situation you would never do this! Following encryption we will then decrypt the resulting ciphertext, and (hopefully!) end up with the message we first started with. This program expects two functions to be defined: "encrypt" and "decrypt". We will define those further down the page. Note that this uses the auto-init facility in 1.1.0.

The program sets up a 256 bit key and a 128 bit IV. This is appropriate for the 256-bit AES encryption that we going to be doing in CBC mode. Make sure you use the right key and IV length for the cipher you have selected, or it will go horribly wrong!! The IV should be random for CBC mode.

We've also set up a buffer for the ciphertext to be placed in. It is important to ensure that this buffer is sufficiently large for the expected ciphertext or you may see a program crash (or potentially introduce a security vulnerability into your code). Note: The ciphertext may be longer than the plaintext (e.g. if padding is being used).

We're also going to need a helper function to handle any errors. This will simply dump any error messages from the OpenSSL error stack to the screen, and then abort the program.

So now that we have set up the program we need to define the "encrypt" function. This will take as parameters the plaintext, the length of the plaintext, the key to be used, and the IV. We'll also take in a buffer to put the ciphertext in (which we assume to be long enough), and will return the length of the ciphertext that we have written.

Encrypting consists of the following stages:

Setting up a context

Initialising the encryption operation

Providing plaintext bytes to be encrypted

Finalising the encryption operation

During initialisation we will provide an EVP_CIPHER object. In this case we are using EVP_aes_256_cbc(), which uses the AES algorithm with a 256-bit key in CBC mode. Refer to EVP#Working with Algorithms and Modes for further details.

Finally we need to define the "decrypt" operation. This is very similar to encryption and consists of the following stages:
Decrypting consists of the following stages:

Setting up a context

Initialising the decryption operation

Providing ciphertext bytes to be decrypted

Finalising the decryption operation

Again through the parameters we will receive the ciphertext to be decrypted, the length of the ciphertext, the key and the IV. We'll also receive a buffer to place the decrypted text into, and return the length of the plaintext we have found.

Note that we have passed the length of the ciphertext. This is required as you cannot use functions such as "strlen" on this data - its binary! Similarly, even though in this example our plaintext really is ASCII text, OpenSSL does not know that. In spite of the name plaintext could be binary data, and therefore no NULL terminator will be put on the end (unless you encrypt the NULL as well of course).

OpenSSL uses PKCS padding by default. If the mode you are using allows you to change the padding, then you can change it with EVP_CIPHER_CTX_set_padding. From the man page:

EVP_CIPHER_CTX_set_padding() enables or disables padding. By default encryption operations are padded using standard block padding and the padding is checked and removed when decrypting. If the pad parameter is zero then no padding is performed, the total amount of data encrypted or decrypted must then be a multiple of the block size or an error will occur...

PKCS padding works by adding n padding bytes of value n to make the total length of the encrypted data a multiple of the block size. Padding is always added so if the data is already a multiple of the block size n will equal the block size. For example if the block size is 8 and 11 bytes are to be encrypted then 5 padding bytes of value 5 will be added...

If padding is disabled then the decryption operation will only succeed if the total amount of data decrypted is a multiple of the block size.

Questions regarding how to use the EVP interfaces from a C++ program arise on occasion. Generally speaking, using the EVP interfaces from a C++ program is the same as using them from a C program.

You can download a sample program using EVP symmetric encryption and C++11 called evp-encrypt.cxx. The sample uses a custom allocator to zeroize memory, C++ smart pointers to manage resources, and provides a secure_string using basic_string and the custom allocator. You need to use g++ -std=c++11 ... to compile it because of std::unique_ptr.

You should also ensure you configure an build with -fexception to ensure C++ exceptions pass as expected through C code. And you should avoid other flags, like -fno-exceptions and -fno-rtti.

The program's main simply encrypts and decrypts a string using AES-256 in CBC mode:

Worthy of mention here is the XTS mode (e.g. EVP_aes_256_xts()). This works in exactly the same way as shown above, except that the "tweak" is provided in the IV parameter. A further "gotcha" is that XTS mode expects a key which is twice as long as normal. Therefore EVP_aes_256_xts() expects a key which is 512-bits long.